How do I bake flat textures using cycles?
To bake texture in Cycles without having any influence from objects and lights in the scene you need to choose Diffuse Color in Bake Type. The result is similar to baking shadeless material in Blender Internal.
From Cycles Render Baking documentation page: Render Baking

Diffuse Color/Direct/Indirect
Bakes the diffuse pass of a material. Diffuse Color is a property of the surface and independent of sampling refinement.